Numerous components, some having complex geometric structures, are produced by placing curable materials such as epoxy or polyester resins in a mold and then curing the material.
Examples of such processes are RTM (resin transfer molding) and VRTM (vacuum-assisted resin transfer molding). Resin transfer molding is a method for the production of molded parts from thermosets and elastomers. In this method, in contrast to the pressing process, the molding compound is injected using a plunger from a usually heated prechamber via distribution channels into the mold cavity, where it is cured under application of heat and pressure. Formaldehyde resins (phenol resins or aminoplast resins) and reactive resins (polyesters such as PET or epoxy resins) with small filler particles and elastomers can be used as molding compounds.
At the start of a cycle, the prechamber contains a preplasticized and dosed molding compound. The mold is first closed. The molding compound is then injected into the mold and left therein for a specified period. During this period, referred to as the residence time, the molding compound is reacted or vulcanized. The residence time depends on a variety of factors (resin type, filler, processing time, and temperature). When the residence time is completed, the mold can be opened. The previous filled molding compound is now solid (cured) and is now referred to as the molded part. The molded part can now be removed from the mold. This is followed by cleaning of the mold, and a new cycle can then begin.
In this case, the amount of the molding compound required for pressing and repressing should always be greater than the volume of the final molded part so that the mold is completely filled. This ensures that the molded part will be fully formed and that no air will be included. The excess molding compound remaining in the prechamber, also referred to as the residual cake, must be removed before the start of the new cycle and replaced by fresh molding compound.
In order to prevent air inclusions, the cavity (mold cavity) is usually evacuated.
In processing of long fibers or semifinished fiber materials as well (prewovens/preforms), said fiber materials are first placed in the mold and then overmolded with the molding compound. In this case as well, it is generally preferable to additionally evacuate the cavity (mold cavity).
“Prewoven methods” can be classified according to the number and configuration of resin injections. In the following, insertion of the resin into the semifinished fiber material is referred to as injection, regardless of how the pressure gradient is produced.                Point injection: The resin is injected at only one site into the semifinished material. In point injection, the flow front may include air, which leads to flaws.        Multiple point injection: The mold can be filled with resin more quickly when multiple injection sites are used. Inclusion of air can be prevented by skilled positioning.        Line injection: In line injection, injection is carried out linearly on the edge of the mold instead of at one point. This can be advantageous in the case of highly elongated components, as the material must flow only through the shorter edge length.        Flow canal injection: The resin is injected via a broad channel located above or below the semifinished fiber material.        Cascade injection: In order to keep the pressure gradient low, multiple injection sites are configured in the direction of the flow front. In this process, however, it is necessary to open and close the injection tubes along the flow front.        
Known mold types are hard molds, soft molds, and mixed molds.
Resins having a low viscosity are used as injection resins. This keeps flow resistance during flowing through the mold low and means that smaller differences in pressure are needed for filling. Reactive resins for RTM methods are sold as special injection resins composed of a resin and curing components. Low-reactivity resin systems can be mixed prior to injection. When high-reactivity resin systems are used, the resin and curing agents can only be mixed directly in the infusion line or in the mold. This makes shorter cycle times possible. Methods in which the injection resin components are not mixed until immediately before injection are known as RIM (reaction injection molding) methods.
Further details can be found in Rompp's Chemistry Lexicon, specifically under the entry “Injection molding” (2013, Georg Thieme Publishing House, document ID No. RD-19-03499, last updated: July 2011).
For constructing a piece, for example the (half) blade of a rotor installed in a wind turbine, glass fiber mats are used that are inserted as partial layers into a suitably configured mold. After this, the layers are bonded with a resin and cured in the mold to produce a fiber-reinforced polymer or a glass fiber-reinforced plastic.
In order to ensure simple and non-destructive demolding, the mold, which constitutes the negative and optionally also the positive impression of the body to be constructed, must be prepared with an anti-adhesive material that is applied to the mold before forming the layers.
Release agents such as polyvinyl alcohol or silicone waxes are frequently used for this purpose. Release agents based on silane or siloxane, such as the products of the Frekote series produced by Henkel, are also known. Moreover, PTFE-coated glass fabric is used that is applied to the mold in the form of an adhesive tape and replaces the release agent.
The release agent is applied in a homogeneous layer, and this layer must be absolutely smooth so that the outer surface of the body is also smooth.
Conventionally-used liquid release agents are solvent-based and require drying and curing times of 20 to 30 min each. The application of the release agent also takes 20 to 30 min.
Depending on the user, new release agents must again be provided before each production cycle, thus resulting in a downtime of 1.5 h before each cycle.
Another drawback is that the release agents are partially transferred to the component, which makes immediately following further processing, such as coating, difficult. The release agent must first be removed, which also takes time.
It also known that the separating effect of such release agents is not 100%. Because the resin has no direct contact with the mold, a small amount of resin is deposited in some areas after each additional demolding cycle. This effect is so strongly cumulative that in severe cases, the mold has to be ground and polished after 200 cycles, as the demolded components otherwise will no longer have the required accuracy of fit.
Some of the release agents used are based on organic solvents, which evaporate on drying and contaminate the ambient air. In certain cases, extra safety measures must be taken for this reason in order to minimize the risk of fire or explosion and health hazards.
One alternative that is not widespread is the method of lining the component with PTFE-coated (glass) fabric adhesive tapes. Depending on their quality, such tapes do not have to be replaced as often and show favorable release. This primarily saves time, which can be used for further production cycles.
The process of applying the fabric adhesive tape in three-dimensional form is disadvantageous because care must be taken to prevent, to the extent possible, the formation of irregularities such as air bubbles under the adhesive tape, as well as the formation of overlapping adhesive tape edges or wrinkles.
This is made extremely difficult by the stiffness and poor flexibility of the PTFE-treated glass fabric carrier.
In the production of a PTFE-coated fabric, an extremely wide bale is ordinarily moistened on its upper and lower sides with PTFE and then later cut into multiple rolls of the desired width. This leaves no PTFE on the cut edges of the rolls. This in turn has the result that the fabric and thus the adhesive tape in the mold become saturated with liquid resin, causing a reduction in the separating effect or the number of cycles required before the tape must be completely replaced.
There are considerable differences in the quality of PTFE-treated glass fabric carriers. It is virtually impossible to prevent so-called microtears in the PTFE layer, into which the resin can penetrate. The frequency of occurrence of the tears depends on quality. These tears cause the adhesive tape to become saturated with resin in the center, which leads to a reduction in service life, as discussed in the above paragraph.
The glass fiber fabric (like any fabric) also cannot be prevented from fraying at the cut edges, leaving individual fibers remaining in the mold. When the component is demolded, this causes the fiber, and partially the adhesive tape, to be torn out as well. Once a defect of this type occurs, it grows with corresponding speed, so that the resulting gap must be patched. As the newly created edges are also sensitive, this constitutes a self-deteriorating process.
A fabric adhesive tape coated with PTFE therefore also gradually loses its separating effect, so that the tape must be replaced, for example after 30 demoldings.
It is therefore a major drawback for the manufacturer of rotor blades when the adhesive tapes cannot be removed from the mold without leaving residues of the adhesive compound on the mold.
Residue-free removability of the adhesive tape is one of the essential requirements that must be met by the adhesive tape.
The adhesive compounds used are ordinarily provided with silicone adhesives. As silicone-based pressure-sensitive adhesive compounds sometimes show sharp increases in adhesive strength, strong forces develop on detachment. If the anchoring to the carrier is poor, the adhesive tape can detach from the carrier and therefore remain on the mold. The resulting residues must be removed by laborious manual means.
The increase in adhesion during storage of the adhesive composite is referred to by the person skilled in the art as an increase in bonding strength, and is caused by interactions between the adhesive and the coating substrate.
In some variants of the adhesive tape, the adhesive compound is properly anchored, but the PTFE layer detaches when the adhesive tape is removed from the glass fabric, which also results in considerable amounts of residue.
Although silicone adhesive tapes already show a favorable separating effect, the mold is also provided with a further layer of release agents before being lined with adhesive tape in order to optimize protection of the mold. After the adhesive tape, the release agent then serves as a second protective layer against penetrating resin in the event of a hole somewhere in the adhesive tape, but need be applied only once at the beginning of the process.
This also means that the adhesive tape must adhere to a substrate to which a release agent has been applied. Silicone-based adhesive compounds are well-suited for this purpose, but have the above-described drawback of sharply increasing adhesive strength in the mold during the service life of the tape, which can lead to residues or even delamination of the adhesive tape.
If the adhesive tape is extremely thin, it will tear on removal because of the high adhesive strength, prolonging the replacement process. If a thicker adhesive tape is selected, this problem is reduced. However, it also more difficult to insert thicker adhesives into 3-D molds.
The object of the present invention is to provide a method for molding a part composed of curable material layers in a mold, which is optimized with respect to its cycle steps by means of an improved adhesive tape that is used between the material layers and the mold.
This object is achieved by means of the method described in the main claim. The dependent claims relate to advantageous improvements in the subject matter of the invention.